Electro-optic displays

ABSTRACT

A thin film transistor (TFT) backplane comprising a plurality of electrodes. Each of the plurality of electrodes is coupled to circuitry comprising: a first thin film transistor (TFT) coupled to the electrode for transmitting waveforms to the electrode, and a second TFT coupled to the electrode for discharging remnant charges from the electrode, wherein the second TFT is activated subsequent to the first TFT being deactivated.

REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. patent application Ser. No.15/992,363 filed on May 30, 2018 and claiming priority to U.S. PatentApplication No. 62/512,212 filed on May 30, 2017. This application isalso related to U.S. patent application Ser. No. 15/015,822 filed onFeb. 4, 2016 (Publication No. 2016/0225322); U.S. patent applicationSer. No. 15/014,236 filed on Feb. 3, 2016 (Publication No.2016/0225321); and U.S. patent application Ser. No. 15/266,554 filed onSep. 15, 2016 (Publication No. 2017/0076672).

All of the above-listed applications are incorporated by reference intheir entireties.

SUBJECT OF THE INVENTION

The subject matter disclosed herein relates to means and methods todrive electro-optic displays. Specifically, the subject matter isrelated to backplane designs for electro-optic displays and methods fordriving and/or discharging such displays.

BACKGROUND

Electrophoretic displays or EPDs are commonly driven by so-calledDC-balanced waveforms. DC-balanced waveforms have been proven to improvelong-term usage of EPDs by reducing severe hardware degradations andeliminating other reliability issues. However, the DC-balance waveformconstraint limits the set of possible waveforms that are available todrive the EPD display, making it difficult or sometimes impossible toimplement advantageous features via a waveform mode. For example, whenimplementing a “flash-less” white-on-black display mode, excessive whiteedge accumulation may become visible when gray-tones that havetransitioned to black are next to a non-flashing black background. Toclear such edges, a DC-imbalanced drive scheme may have worked well, butsuch drive scheme requires breaking the DC-balance constraint. However,DC-imbalanced drive schemes or waveforms can cause hardware degradationsover time which shortens display devices' lifetime. As such, thereexists a need to design electro-optic displays capable of operating withDC-imbalanced waveforms or drive schemes without suffering hardwaredegradations.

SUMMARY

According to one embodiment of the subject matter presented herein, anelectro-optic display may comprise an electrophoretic materialconfigured for displaying images, and an active component coupled to theelectrophoretic material for discharging charges within theelectrophoretic material.

In another embodiment in accordance with the subject matter disclosedherein, an electro-optic display may comprise an electrophoreticmaterial configured for displaying images, and a passive componentcoupled to the electrophoretic material for discharging charges withinthe electrophoretic material.

In yet another embodiment, a method for discharging remnant voltage froman electro-optic display, where the electro-optic display may have anelectrophoretic material configured to display images, theelectrophoretic material positioned between a pixel electrode and acommon electrode, a thin-film-transistor (TFT) coupled to theelectrophoretic material and configured for discharging charges from theelectrophoretic material, and a select line coupled to the TFT toactivate the TFT, the method may comprise supplying a voltage throughthe select line to activate the TFT to create a conductive path to theelectrophoretic material through the TFT, and discharging the chargeswithin the electrophoretic material through the conductive path.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is one embodiment of an equivalent circuit of a display pixel inaccordance with the subject matter presented herein;

FIGS. 2A and 2B are graphs illustrating graytone and ghosting shifts ofa display due to shifts in TFT performance;

FIG. 3 is an exemplary pixel design in accordance with the subjectmatter presented herein to enable the use of post-drive dischargingwithout introducing optical shifts;

FIG. 4 is another pixel design in accordance with the subject matterpresented herein to enable the use of post-drive discharging withoutintroducing optical shifts;

FIG. 5 are voltage sequences for an active update followed by adischarge;

FIG. 6 is another pixel design in accordance with the subject matterpresented herein to enable the use of post-drive discharging withoutintroducing optical shifts;

FIG. 7 is yet another pixel design in accordance with the subject matterpresented herein to enable the use of post-drive discharging withoutintroducing optical shifts; and

FIG. 8 are voltage sequences for an active update followed bydischarging.

DETAILED DESCRIPTION

The subject matter disclosed herein relates to improving electro-opticdisplay durabilities. Specifically, it is related to improving opticalperformance shifts such as mitigating gray-tone shifts and ghostingshifts caused by component stresses.

The term “electro-optic”, as applied to a material or a display, is usedherein in its conventional meaning in the imaging art to refer to amaterial having first and second display states differing in at leastone optical property, the material being changed from its first to itssecond display state by application of an electric field to thematerial. Although the optical property is typically color perceptibleto the human eye, it may be another optical property, such as opticaltra.nsmission, reflectance, luminescence or, in the case of displaysintended for machine reading, pseudo-color in the sense of a change inreflectance of electromagnetic wavelengths outside the visible range.

The terms “bistable” and “bistability” are used herein in theirconventional meaning in the art to refer to displays comprising displayelements having first and second display states differing in at leastone optical property, and such that after any given element has beendriven, by means of an addressing pulse of finite duration, to assumeeither its first or second display state, after the addressing pulse hasterminated, that state will persist for at least several times, forexample at least four times, the minimum duration of the addressingpulse required to change the state of the display element. It is shownin U.S. Pat. No. 7,170,670 that some particle-based electrophoreticdisplays capable of gray scale are stable not only in their extremeblack. and white states but also in their intermediate gray states, andthe same is true of some other types of electro-optic displays. Thistype of display is properly called “multi-stable” rather than bistable,although for convenience the term “bistable” may be used herein to coverboth bistable and multi-stable displays.

The term “gray state” is used herein in its conventional meaning in theimaging art to refer to a state intermediate two extreme optical statesof a pixel, and does not necessarily imply a black-white transitionbetween these two extreme states, For example, several of the F Inkpatents and published applications referred to below describeelectrophoretic displays in which the extreme states are white and deepblue, so that an intermediate “gray state” would actually be pale blue.Indeed, as already mentioned, the change in optical state may not be acolor change at all. The terms “black” and “white” may be usedhereinafter to refer to the two extreme optical states of a display, andshould be understood as normally including extreme optical states whichare not strictly black and white, for example, the aforementioned whiteand dark blue states. The term “monochrome” may be used hereinafter todenote a display or drive scheme which only drives pixels to their twoextreme optical states with no intervening gray states.

The term “pixel” is used herein in its conventional meaning in thedisplay art to mean the smallest unit of a display capable of generatingall the colors which the display itself can show. In a full colordisplay, typically each pixel is composed of a plurality of sub-pixelseach of which can display less than all the colors which the displayitself can show. For example, in most conventional full color displays,each pixel is composed of a red sub-pixel, a green sub-pixel, a bluesub-pixel, and optionally a white sub-pixel, with each of the sub-pixelsbeing capable of displaying a range of colors from black to thebrightest version of its specified color.

Several types of electro-optic displays are known. One type ofelectro-optic display is a rotating bichromal member type as described,for example, in U.S. Pat. Nos. 5,808,783; 5,777,782; 5,760,761;6,054,071 6,055,091; 6,097,531; 6,128,124; 6,137,467; and 6,147,791(although this type of display is often referred to as a “rotatingbichromal ball” display, the term “rotating bichromal member” ispreferred as more accurate since in some of the patents mentioned abovethe rotating members are not spherical). Such a display uses a largenumber of small bodies (typically spherical or cylindrical) which havetwo or more sections with differing optical characteristics, and aninternal dipole. These bodies are suspended within liquid-filledvacuoles within a matrix, the vacuoles being filled with liquid so thatthe bodies are free to rotate. The appearance of the display is changedby applying an electric field thereto, thus rotating the bodies tovarious positions and varying which of the sections of the bodies isseen through a viewing surface. This type of electro-optic medium istypically bistable.

Another type of electro-optic display uses an electrochromic medium, forexample an electrochromic medium in the form of a nanochromic filmcomprising an electrode formed at least in part from a semi-conductingmetal oxide and a plurality of dye molecules capable of reversible colorchange attached to the electrode; see, for example O'Regan, B., et al.,Nature 1991, 353, 737; and Wood, D., Information Display, 18(3), 24(March 2002). See also Bach, U., et al., Adv. Mater., 2002, 1401), 845.Nanochromic films of this type are also described, for example, in U.S.Pat. Nos. 6,301,038; 6,870,657; and 6,950,220. This type of medium isalso typically bistable.

Another type of electro-optic display is an electro-wetting displaydeveloped by Philips and described in Hayes, R A., et al., “Video-SpeedElectronic Paper Based on Electrowetting”, Nature, 425, 383-385 (2003).It is shown in U.S. Pat. No. 7,420,549 that such electro-wettingdisplays can be made bistable.

One type of electro-optic display, which has been the subject of intenseresearch and development for a number of years, is the particle-basedelectrophoretic display, in which a plurality of charged particles movethrough a fluid under the influence of an electric field.Electrophoretic displays can have attributes of good brightness andcontrast, wide viewing angles, state bistability, and low powerconsumption when compared with liquid crystal displays.

As noted above, electrophoretic media require the presence of a fluid.In most prior art electrophoretic media, this fluid is a liquid, butelectrophoretic media can be produced using gaseous fluids; see, forexample, Kitamura, T., et al., “Electrical toner movement for electronicpaper-like display”, IDW Japan, 2001, Paper HCS1-1, and Yamaguchi, Y.,et al., “Toner display using insulative particles chargedtriboelectrically”, IDW Japan, 2001, Paper AMD4-4). See also U.S. Pat.Nos. 7,321,459 and 7,236,291. Such gas-based electrophoretic mediaappear to be susceptible to the same types of problems due to particlesettling as liquid-based electrophoretic media, when the media are usedin an orientation which permits such settling, for example in a signwhere the medium is disposed in a vertical plane. Indeed, particlesettling appears to be a more serious problem in gas-basedelectrophoretic media than in liquid-based ones, since the lowerviscosity of gaseous suspending fluids as compared with liquid onesallows more rapid settling of the electrophoretic particles.

Numerous patents and applications assigned to or in the names of theMassachusetts Institute of Technology (MIT) and E Ink Corporationdescribe various technologies used in encapsulated electrophoretic andother electro-optic media. Such encapsulated media comprise numeroussmall capsules, each of which itself comprises an internal phasecontaining electrophoretically-mobile particles in a fluid medium, and acapsule wall surrounding the internal phase. Typically, the capsules arethemselves held within a polymeric binder to form a coherent layerpositioned between two electrodes. The technologies described in thesepatents and applications include:

-   -   (a) Electrophoretic particles, fluids and fluid additives; see        for example U.S. Pat. Nos. 7,002,728 and 7,679,814;    -   (b) Capsules, binders and encapsulation processes; see for        example U.S. Pat. Nos. 6,922,276 and 7,411,719;    -   (c) Films and sub-assemblies containing electro-optic materials;        see for example U.S. Pat. Nos. 6,982,178 and 7,839,564;    -   (d) Backplanes, adhesive layers and other auxiliary layers and        methods used in displays; see for example U.S. Pat. Nos.        D485,294; 6,124,851; 6,130,773; 6,177,921; 6,232,950; 6,252,564;        6,312,304; 6,312,971; 6,376,828; 6,392,786; 6,413,790;        6,422,687; 6,445,374; 6,480,182; 6,498,114; 6,506,438;        6,518,949; 6,521,489; 6,535,197; 6,545,291; 6,639,578;        6,657,772; 6,664,944; 6,680,725; 6,683,333; 6,724,519;        6,750,473; 6,816,147; 6,819,471; 6,825,068; 6,831,769;        6,842,167; 6,842,279; 6,842,657; 6,865,010; 6,873,452;        6,909,532; 6,967,640; 6,980,196; 7,012,735; 7,030,412;        7,075,703; 7,106,296; 7,110,163; 7,116,318; 7,148,128;        7,167,155; 7,173,752; 7,176,880; 7,190,008; 7,206,119;        7,223,672; 7,230,751; 7,256,766; 7,259,744; 7,280,094;        7,301,693; 7,304,780; 7,327,511; 7,347,957; 7,349,148;        7,352,353; 7,365,394; 7,365,733; 7,382,363; 7,388,572;        7,401,758; 7,442,587; 7,492,497; 7,535,624; 7,551,346;        7,554,712; 7,583,427; 7,598,173; 7,605,799; 7,636,191;        7,649,674; 7,667,886; 7,672,040; 7,688,497; 7,733,335;        7,785,988; 7,830,592; 7,843,626; 7,859,637; 7,880,958;        7,893,435; 7,898,717; 7,905,977; 7,957,053; 7,986,450;        8,009,344; 8,027,081; 8,049,947; 8,072,675; 8,077,141;        8,089,453; 8,120,836; 8,159,636; 8,208;193; 8,237,892;        8,238,021; 8,362,488; 8,373,211; 8,389,381; 8,395,836;        8,437,069; 8,441,414; 8,456,589; 8,498,042; 8,514,168;        8,547,628; 8,576,162; 8,610,988; 8,714,780; 8,728,266;        8,743,077; 8,754,859; 8,797,258; 8,797,633; 8,797,636;        8,830,560; 8,891,155; 8,969,886; 9,147,364; 9,025,234;        9,025,238; 9,030,374; 9,140,952; 9,152,003; 9,152,004;        9,201,279; 9,223,164; 9,285,648; and 9,310,661; and U.S. Patent        Applications Publication Nos. 2002/0060321; 2004/0008179;        2004/0085619; 2004/0105036; 2004/0112525; 2005/0122306;        2005/0122563; 2006/0215106; 2006/0255322; 2007/0052757;        2007/0097489; 2007/0109219; 2008/0061300; 2008/0149271;        2009/0122389; 2009/0315044; 2010/0177396; 2011/0140744;        2011/0187683; 2011/0187689; 2011/0292319; 2013/0250397;        2013/0278900; 2014/0078024; 2014/0139501; 2014/0192000;        2014/0210701; 2014/0300837; 2014/0368753; 2014/0376164;        2015/0171112; 2015/0205178; 2015/0226986; 2015/0227018;        2015/0228666; 2015/0261057; 2015/0356927; 2015/0378235;        2016/077375; 2016/0103380; and 2016/0187759; and International        Application Publication No. WO 00/38000; European Patents Nos.        1,099,207 B1 and 1,145,072 B1;    -   (e) Color formation and color adjustment; see for example U.S.        Pat. Nos. 6,017,584; 6,664,944; 6,864,875; 7,075,502; 7,167,155;        7,667,684; 7,791,789; 7,956,841; 8,040,594; 8,054,526;        8,098,418; 8,213,076; and 8,363,299; and U.S. Patent        Applications Publication Nos. 2004/0263947; 2007/0109219;        2007/0223079; 2008/0023332; 2008/0043318; 2008/0048970;        2009/0004442; 2009/0225398; 2010/0103502; 2010/0156780;        2011/0164307; 2011/0195629; 2011/0310461; 2012/0008188;        2012/0019898; 2012/0075687; 2012/0081779; 2012/0134009;        2012/0182597; 2012/0212462; 2012/0157269; and 2012/0326957;    -   (f) Methods for driving displays; see for example U.S. Pat. Nos.        7,012,600 and 7,453,445;    -   (g) Applications of displays; see for example U.S. Pat. Nos.        7,312,784 and 8,009,348;    -   (h) Non-electrophoretic displays, as described in U.S. Pat. Nos.        6,241,921; 6,950,220; 7,420,549 and 8,319,759; and U.S. Patent        Application Publication No. 2012/0293858;    -   (i) Microcell structures, wall materials, and methods of forming        microcells; see for example U.S. Pat. Nos. 7,072,095 and        9,279,906; and    -   (j) Methods for filling and sealing microcells; see for example        U.S. Pat. Nos. 7,144,942 and 7,715,088.

This application is further related to U.S. Pat. Nos. D485,294;6,124,851; 6,130,773; 6,177,921; 6,232,950; 6,252,564; 6,312,304;6,312,971; 6,376,828; 6,392,786; 6,413,790; 6,422,687; 6,445,374;6,480,182; 6,498,114; 6,506,438; 6,518,949; 6,521,489; 6,535,197;6,545,291; 6,639,578; 6,657,772; 6,664,944; 6,680,725; 6,683,333;6,724,519; 6,750,473; 6,816,147; 6,819,471; 6,825,068; 6,831,769;6,842,167; 6,842,279; 6,842,657; 6,865,010; 6,873,452; 6,909532;6,967,640; 6,980,196; 7,012,735; 7,030,412; 7,075,703; 7,106,296;7,110,163; 7,116,318; 7,148,128; 7,167,155; 7,173,752; 7,176,880;7,190,008; 7,206,119; 7,223,672; 7,230,751; 7,256,766; 7,259,744;7,280,094; 7,301,693; 7,304,780; 7,327,511; 7,347,957; 7,349,148;7,352,353; 7,365,394; 7,365,733; 7,382,363; 7,388,572; 7,401,758;7,442,587; 7,492,497; 7,535,624; 7,551,346; 7,554,712; 7,583,427;7,598,173; 7,605,799; 7,636,191; 7,649,674; 7,667,886; 7,672,040;7,688,497; 7,733,335; 7,785,988; 7,830,592; 7,843,626; 7,859,637;7,880,958; 7,893,435; 7,898,717; 7,905,977; 7,957,053; 7,986,450;8,009,344; 8,027,081; 8,049,947; 8,072,675; 8,077,141; 8,089,453;8,120,836; 8,159,636; 8,208,193; 8,237,892; 8,238,021; 8,362,488;8,373,211; 8,389,381; 8,395,836; 8,437,069; 8,441,414; 8,456,589;8,498,042; 8,514,168; 8,547,628; 8,576,162; 8,610,988; 8,714,780;8,728,266; 8,743,077 8,754,859; 8,797,258; 8,797,633; 8,797,636;8,830,560; 8,891,155; 8,969,886; 9,147,364; 9,025,234; 9,025,238;9,030,374; 9,140,952; 9,152,003; 9,152,004; 9,201,279; 9,223,164;9,285,648; and 9,310,661; and U.S. Patent Applications Publication Nos.2002/0060321; 2004/0008179; 2004/0085619; 2004/0105036; 2004/0112525;2005/0122306; 2005/0122563; 2006/0215106; 2006/0255322; 2007/0052757;2007/0097489; 2007/0109219; 2008/0061300; 2008/0149271; 2009/0122389;2009/0315044; 2010/0177396; 2011/0140744; 2011/0187683; 2011/0187689;2011/0292319; 2013/0250397; 2013/0278900; 2014/0078024; 2014/0139501;2014/0192000; 2014/0210701; 2014/0300837; 2014/0368753; 2014/0376164;2015/0171112; 2015/0205178; 2015/0226986; 2015/0227018; 2015/0228666;2015/0261057; 2015/0356927; 2015/0378235; 2016/077375; 2016/0103380; and2016/0187759; and International Application Publication No. WO 00/38000;European Patents Nos. 1,099,207 B1 and 1,145,072 B1; all of theabove-listed applications are incorporated by reference in theirentireties.

This application is also related to U.S. Pat. Nos. 5,930,026; 6,445,489;6,504,524; 6,512,354; 6,531,997; 6,753,999; 6,825,970; 6,900,851;6,995,550; 7,012,600; 7,023,420; 7,034,783; 7,061,166; 7,061,662;7,116,466; 7,119,772; 7,177,066; 7,193,625; 7,202,847; 7,242,514;7,259,744; 7,304,787; 7,312,794; 7,327,511; 7,408,699; 7,453,445;7,492,339; 7,528,822; 7,545,358; 7,583,251; 7,602,374; 7,612,760;7,679,599; 7,679,813; 7,683,606; 7,688,297; 7,729,039; 7,733,311;7,733,335; 7,787,169; 7,859,742; 7,952,557; 7,956,841; 7,982,479;7,999,787; 8,077,141; 8,125,501; 8,139,050; 8,174,490; 8,243,013;8,274,472; 8,289,250; 8,300,006; 8,305,341; 8,314,784; 8,373,649;8,384,658; 8,456,414; 8,462,102; 8,537,105; 8,558,783; 8,558,785;8,558,786; 8,558,855; 8,576,164; 8,576,259; 8,593,396; 8,605,032;8,643,595; 8,665,206; 8,681,191; 8,730,153; 8,810,525; 8,928,562;8,928,641; 8,976,444; 9,013,394; 9,019,197; 9,019,198; 9,019,318;9,082,352; 9,171,508; 9,218,773; 9,224,338; 9,224,342; 9,224,344;9,230,492 9,251,736; 9,262,973; 9,269,311; 9,299,294; 9,373,289;9,390,066; 9,390,661; and 9,412,314; and U.S. Patent ApplicationsPublication Nos. 2003/0102858; 2004/0246562; 2005/0253777; 2007/0070032;2007/0076289; 2007/0091418; 2007/0103427; 2007/0176912; 2007/0296452;2008/0024429; 2008/0024482; 2008/0136774; 2008/0169821; 2008/0218471;2008/0291129; 2008/0303780; 2009/0174651; 2009/0195568; 2009/0322721;2010/0194733; 2010/0194789; 2010/0220121; 2010/0265561; 2010/0283804;2011/0063314; 2011/0175875; 2011/0193840; 2011/0193841; 2011/0199671;2011/0221740; 2012/0001957; 2012/0098740; 2013/0063333; 2013/0194250;2013/0249782; 2013/0321278; 2014/0009817; 2014/0085355; 2014/0204012;2014/0218277; 2014/0240210; 2014/0240373; 2014/0253425; 2014/0292830;2014/0293398; 2014/0333685; 2014/0340734; 2015/0070744; 2015/0097877;2015/0109283; 2015/0213749; 2015/0213765; 2015/0221257; 2015/0262255;2016/0071465; 2016/0078820; 2016/0093253; 2016/0140910; and2016/0180777; all of the above-listed applications are incorporated byreference in their entireties.

Many of the aforementioned patents and applications recognize that thewalls surrounding the discrete microcapsules in an encapsulatedelectrophoretic medium could be replaced by a continuous phase, thusproducing a so-called polymer-dispersed electrophoretic display, inwhich the electrophoretic medium comprises a plurality of discretedroplets of an electrophoretic fluid and a continuous phase of apolymeric material, and that the discrete droplets of electrophoreticfluid within such a polymer-dispersed electrophoretic display may beregarded as capsules or microcapsules even though no discrete capsulemembrane is associated with each individual droplet; see for example,the aforementioned U.S. Pat. No. 6,866,760, Accordingly, for purposes ofthe present application, such polymer-dispersed electrophoretic mediaare regarded as sub-species of encapsulated electrophoretic media.

A related type of electrophoretic display is a so-called “microcellelectrophoretic display”. In a microcell electrophoretic display, thecharged particles and the fluid are not encapsulated withinmicrocapsules but instead are retained within a plurality of cavitiesformed within a carrier medium, typically a polymeric film. See, forexample, U.S. Pat. Nos. 6,672,921 and 6,788,449, both assigned to SipixImaging, Inc.

Although electrophoretic media are often opaque (since, for example, inmany electrophoretic media, the particles substantially blocktransmission of visible light through the display) and operate in areflective mode, many electrophoretic displays can be made to operate ina so-called “shutter mode” in which one display state is substantiallyopaque and one is light-transmissive. See, for example, U.S. Pat. Nos.5,872,552; 6,130,774; 6,144,361; 6,172,798; 6,271,823; 6,225,971; and6,184,856. Dielectrophoretic displays, which are similar toelectrophoretic displays but rely upon variations in electric fieldstrength, can operate in a similar mode; see U.S. Pat. No. 4,418,346.Other types of electro-optic displays may also be capable of operatingin shutter mode. Electro-optic media operating in shutter mode may beuseful in multi-layer structures for full color displays; in suchstructures, at least one layer adjacent the viewing surface of thedisplay operates in shutter mode to expose or conceal a second layermore distant from the viewing surface.

An encapsulated electrophoretic display typically does not suffer fromthe clustering and settling failure mode of traditional electrophoreticdevices and provides further advantages, such as the ability to print orcoat the display on a wide variety of flexible and rigid substrates.(Use of the word “printing” is intended to include all forms of printingand coating, including, but without limitation: pre-metered coatingssuch as patch die coating, slot or extrusion coating, slide or cascadecoating, curtain coating; roll coating such as knife over roll coating,forward and reverse roll coating; gravure coating; dip coating; spraycoating; meniscus coating; spin coating; brush coating; air knifecoating; silk screen printing processes; electrostatic printingprocesses; thermal printing processes; ink jet printing processes;electrophoretic deposition (See U.S. Pat. No. 7,339,715); and othersimilar techniques.) Thus, the resulting display can be flexible.Further, because the display medium can be printed, using a variety ofmethods, the display itself can be made inexpensively.

Other types of electro-optic materials may also be used in the presentinventi on.

An electrophoretic display normally comprises a layer of electrophoreticmaterial and at least two other layers disposed on opposed sides of theelectrophoretic material, one of these two layers being an electrodelayer. In most such displays both the layers are electrode layers, andone or both of the electrode layers are patterned to define the pixelsof the display. For example, one electrode layer may be patterned intoelongate row electrodes and the other into elongate column electrodesrunning at right angles to the row electrodes, the pixels being definedby the intersections of the row and column electrodes. Alternatively,and more commonly, one electrode layer has the form of a singlecontinuous electrode and the other electrode layer is patterned into amatrix of pixel electrodes, each of which defines one pixel of thedisplay. In another type of electrophoretic display, which is intendedfor use with a stylus, print head or similar movable electrode separatefrom the display, only one of the layers adjacent the electrophoreticlayer comprises an electrode, the layer on the opposed side of theelectrophoretic layer typically being a protective layer intended toprevent the movable electrode damaging the electrophoretic layer.

In yet another embodiment, such as described in U.S. Pat. No. 6,704,133,electrophoretic displays may be constructed with two continuouselectrodes and an el ectrophoretic layer and a photoelectrophoreticlayer between the electrodes. Because the photoelectrophoretic materialchanges resistivity with the absorption of photons, incident light canbe used to alter the state of the electrophoretic medium. Such a deviceis illustrated in FIG. 1. As described in U.S. Pat. No. 6,704,133, thedevice of FIG. 1 works best when driven by an emissive source, such asan LCD display, located on the opposed side of the display from theviewing surface. In some embodiments, the devices of U.S. Pat. No.6,704,133 incorporated special barrier layers between the frontelectrode and the photoelectrophoretic material to reduce “darkcurrents” caused by incident light from the front of the display thatleaks past the reflective electro-optic media.

The aforementioned U.S. Pat. No. 6,982,178 describes a method ofassembling a solid electro-optic display (including an encapsulatedelectrophoretic display) which is well adapted for mass production,Essentially, this patent describes a so-called “front plane laminate”(“FPL”) which comprises, in order, a light-transmissiveelectrically-conductive layer; a layer of a solid electro-optic mediumin electrical contact with the electrically-conductive layer; anadhesive layer; and a release sheet. Typically, the light-transmissiveelectrically-conductive layer will be carried on a light-transmissivesubstrate, which is preferably flexible, in the sense that the substratecan be manually wrapped around a drum (say) 10 inches (254 mm) indiameter without permanent deformation. The term “light-transmissive” isused in this patent and herein to mean that the layer thus designatedtransmits sufficient light to enable an observer, looking through thatlayer, to observe the change in display states of the electro-opticmedium, which will normally be viewed through theelectrically-conductive layer and adjacent substrate (if present); incases where the electro-optic medium displays a change in reflectivityat non-visible wavelengths, the term “light-transmissive” should ofcourse be interpreted to refer to transmission of the relevantnon-visible wavelengths. The substrate will typically be a polymericfilm, and will normally have a thickness in the range of about 1 toabout 25 mil (25 to 634 μm), preferably about 2 to about 10 mil (51 to254 μm). The electrically-conductive layer is conveniently a thin metalor metal oxide layer of, for example, aluminum or ITO, or may be aconductive polymer. Poly (ethylene terephthalate) (PET) films coatedwith aluminum or ITO are available commercially, for example as“aluminized Mylar” (“Mylar” is a Registered Trade Mark) from E.I. duPont de Nemours & Company, Wilmington Del., and such commercialmaterials may be used with good results in the front plane laminate.

The aforementioned U.S. Pat. No. 6,982,178 also describes a method fortesting the electro-optic medium in a front plane laminate prior toincorporation of the front plane laminate into a display. in thistesting method, the release sheet is provided with an electricallyconductive layer, and a voltage sufficient to change the optical stateof the electro-optic medium is applied between this electricallyconductive layer and the electrically conductive layer on the opposedside of the electro-optic medium. Observation of the electro-opticmedium will then reveal any faults in the medium, thus avoidinglaminating faulty electro-optic medium into a display, with theresultant cost of scrapping the entire display, not merely the faultyfront plane laminate.

The aforementioned U.S. Pat. No. 6,982,178 also describes a secondmethod for testing the electro-optic medium in a front plane laminate byplacing an electrostatic charge on the release sheet, thus forming animage on the electro-optic medium. This image is then observed in thesame way as before to detect any faults in the electro-optic medium.

Assembly of an electro-optic display using such a front plane laminatemay be effected by removing the release sheet from the front planelaminate and contacting the adhesive layer with the backplane underconditions effective to cause the adhesive layer to adhere to thebackplane, thereby securing the adhesive layer, layer of electro-opticmedium and electrically-conductive layer to the backplane. This processis well-adapted to mass production since the front plane laminate may bemass produced, typically using roll-to-roll coating techniques, and thencut into pieces of any size needed for use with specific backplanes.

U.S. Pat. No. 7,561,324 describes a so-called “double release sheet”which is essentially a simplified version of the front plane laminate ofthe aforementioned. U.S. Pat. No. 6,982,178. One form of the doublerelease sheet comprises a layer of a solid electro-optic mediumsandwiched between two adhesive layers, one or both of the adhesivelayers being covered by a release sheet. Another form of the doublerelease sheet comprises a layer of a solid electro-optic mediumsandwiched between two release sheets. Both forms of the double releasefilm are intended for use in a process generally similar to the processfor assembling an electro-optic display from a front plane laminatealready described, but involving two separate laminations; typically, ina first lamination the double release sheet is laminated to a frontelectrode to form a front sub-assembly, and then in a second laminationthe front sub-assembly is laminated to a backplane to form the finaldisplay, although the order of these two laminations could be reversedif desired.

U.S. Pat. No. 7,839,564 describes a so-called “inverted front planelaminate”, which is a variant of the front plane laminate described inthe aforementioned U.S. Pat. No. 6,982,178. This inverted front planelaminate comprises, in order, at least one of a light-transmissiveprotective layer and a light-transmissive electrically-conductive layer;an adhesive layer; a layer of a solid electro-optic medium; and arelease sheet. This inverted front plane laminate is used to form anelectro-optic display having a layer of lamination adhesive between theelectro-optic layer and the front electrode or front substrate; asecond, typically thin layer of adhesive may or may not be presentbetween the electro-optic layer and a backplane. Such electro-opticdisplays can combine good resolution with good low temperatureperformance.

The photoelectrophoretic properties of certain pigments were recognizedsome time ago. For example U.S. Pat. No. 3,383,993 discloses aphotoelectrophoretic imaging apparatus that could be used to reproduceprojected images on a medium, typically a transparent electrode, such asITO, The photoelectrophoretic process described in the '993 patent, andother related patents by Xerox Corporation, was not reversible, however,because the photoelectrophoretic process involved thephotoelectrophoretic particles migrating to an “injecting electrode”where they would become attached to the electrode. Because of the lackof reversibility, as well as the cost and complication of the setup,this phenomenon was not commercialized widely.

While displays of the invention are intended to display images for longperiods of time with little to no energy input, the looped displays,described above, can be used to refresh content on the same time scaleas emissive displays, e.g., large format LED displays. Displays of theinvention can display two different images in less than one hour, e.g.,in less than 10 minutes, e.g., in less than five minutes, e.g., in lessthan two minutes. Furthermore, the refresh periods can be staggered,depending upon the use of the display. For example, a transportationschedule may be refreshed every five minutes with an advertisement thatlasts for 30 seconds, whereupon the transportation schedule is returnedfor another five minute period.

In some cases, one way of enabling the use of DC-imbalanced waveforms isdischarging the display module after an active update. Where discharginginvolves short-circuiting the display's imaging film to drain awayresidual charges that builds-up on the imaging film (e.g., a layer ofelectrophoretic material) due to the DC imbalance drive. The use ofupdate Post Drive Discharging (uPDD or UPD to be referred to herein) hassuccessfully demonstrated the reduction in the build-up of residualcharges (as measured by the remnant voltage) and the correspondingmodule polarization that would have resulted in permanent degradation ofthe imaging film due to electrochemistry.

It has now been found that remnant voltage is a more general phenomenonin electrophoretic and other impulse-driven electro-optic displays, bothin cause(s) and effect(s). It has also been found that DC imbalances maycause long-term lifetime degradation of some electrophoretic displays.

There are multiple potential sources of remnant voltage. It is believed(although some embodiments are in no way limited by this belief), that aprimary cause of remnant voltage is ionic polarization within thematerials of the various layers forming the display.

Such polarization occurs in various ways. In a first (for convenience,denoted “Type I”) polarization, an ionic double layer is created acrossor adjacent a material interface. For example, a positive potential atan indium-tin-oxide (“ITO”) electrode may produce a correspondingpolarized layer of negative ions in an adjacent laminating adhesive. Thedecay rate of such a polarization layer is associated with therecombination of separated ions in the lamination adhesive layer. Thegeometry of such a polarization layer is determined by the shape of theinterface, but may be planar in nature.

In a second (“Type II”) type of polarization, nodules, crystals or otherkinds of material heterogeneity within a single material can result inregions in which ions can move or less quickly than the surroundingmaterial. The differing rate of ionic migration can result in differingdegrees of charge polarization within the bulk of the medium, andpolarization may thus occur within a single display component. Such apolarization may be substantially localized in nature or dispersedthroughout the layer.

In a third (“Type III”) type of polarization, polarization may occur atany interface that represents a barrier to charge transport of anyparticular type of ion. One example of such an interface in amicrocavity electrophoretic display is the boundary between theelectrophoretic suspension including the suspending medium and particles(the “internal phase”) and the surrounding medium including walls,adhesives and binders (the “external phase”). In many electrophoreticdisplays, the internal phase is a hydrophobic liquid whereas theexternal phase is a polymer, such as gelatin. Ions that are present inthe internal phase may be insoluble and non-diffusible in the externalphase and vice versa, On the application of an electric fieldperpendicular to such an interface, polarization layers of opposite signwill accumulate on either side of the interface. When the appliedelectric field is removed, the resulting non-equilibrium chargedistribution will result in a measurable remnant voltage potential thatdecays with a relaxation time determined by the mobility of the ions inthe two phases on either side of the interface.

Polarization may occur during a drive pulse. Each image update is anevent that may affect remnant voltage. A positive waveform voltage cancreate a remnant voltage across an electro-optic medium that is of thesame or opposite polarity (or nearly zero) depending on the specificelectro-optic display.

It will be evident from the foregoing discussion that polarization mayoccur at multiple locations within the electrophoretic or otherelectro-optic display, each location having its own characteristicspectrum of decay times, principally at interfaces and at materialheterogeneities. Depending on the placement of the sources of thesevoltages (in other words, the polarized charge distribution) relative tothe electro-active parts (for example, the electrophoretic suspension),and the degree of electrical coupling between each kind of chargedistribution and the motion of the particles through the suspension, orother electro-optic activity, various kinds of polarization will producemore or less deleterious effects. Since an electrophoretic displayoperates by motion of charged particles, which inherently causes apolarization of the electro-optic layer, in a sense a preferredelectrophoretic display is not one in which no remnant voltages arealways present in the display, but rather one in which the remnantvoltages do not cause objectionable electro-optic behavior. ideally, theremnant impulse will be minimized and the remnant voltage will decreasebelow 1 V, and preferably below 0.2 V, within 1 second, and preferablywithin 50 ms, so that that by introducing a minimal pause between imageupdates, the electrophoretic display may affect all transitions betweenoptical states without concern for remnant voltage effects. Forelectrophoretic displays operating at video rates or at voltages below+/−15 V these ideal values should be correspondingly reduced. Similarconsiderations apply to other types of electro-optic display.

To summarize, remnant voltage as a phenomenon is at least substantiallya result of ionic polarization occurring within the display materialcomponents, either at interfaces or within the materials themselves.Such polarizations are especially problematic when they persist on ameso time scale of roughly 50 ms to about an hour or longer. Remnantvoltage can present itself as image ghosting or visual artifacts in avariety of ways, with a degree of severity that can vary with theelapsed times between image updates. Remnant voltage can also create aDC imbalance and reduce ultimate display lifetime. The effects ofremnant voltage therefore may be deleterious to the quality of theelectrophoretic or other electro-optic device and it is desirable tominimize both the remnant voltage itself, and the sensitivity of theoptical states of the device to the influence of the remnant voltage.

In practice, charges built up within an electrophoretic material due topolarization effect described above may be discharged or drained tomitigate the remnant voltage effect. In some embodiment, such dischargemay be performed after an update or drive sequence.

In some embodiments, a post-drive or post-update discharging may beperformed using a readily available thin-film-transistor (TFT) backplane100 for an EPI) and the EPD's controller circuitry, as illustrated inFIG. 1. In use, each display pixel may include a thin film transistorUPD (e.g., TFT_((upd))) 102 that can be configured to provide a certaindegree of electrical conduction such that the display's top plane 106and source (or data) line VS are held at the same voltage potential forsome time (e.g., ground). The above mentioned patent application Ser.No. 15/014,236, which is incorporated herein in its entirety, discussessuch driving method in more detail. The display pixel 100 as illustratedherein, as well as the various embodiments illustrated below, usuallyinclude an electrophoretic material 108 positioned between an pixelelectrode 104 and the top plane 106, where the top plane 106 may includea substrate and a common electrode, and the common electrode can be atransparent conductive layer. Normally, the TFT_((upd)) 102 is designedto function as the pixel controlling transistor for providing ortransmitting driving waveforms to the pixel's pixel electrode 104. Assuch, the TFT_((upd)) 102 is usually configured to operate in aconduction state (i.e., the “ON” state) for a very short amount of timein comparison to the non-conduction state (i.e., the “OFF” state), forexample, in the ratio of more than 1:1000 of “ON” time over “OFF” time.While the use of uPDD will change this ratio to about 1:2 or 1:50depending on the uPDD configurations, which leads to positive biasstress after long terms of usage, in some cases the usage will amount tostress normally caused by tens of thousands of image updates or more.Positive bias stress is known to cause threshold voltage shifts inamorphous silicon TFTs that is permanent. A shift in threshold voltagecan result in behavior changes to the affected TFT and the TFTbackplane, which in turn results in optical shifts in the opticalperformances of the EPD. The optical shift due to uPDD has been observedand is illustrated in FIGS. 2A and 2B. As shown, due to uPDD, displaygray-tone (FIG. 2A) and ghosting shift (FIG. 213) values can increasesignificantly in a two year period after tens of thousands updatecycles.

With using only a single TFT such as the TFT_((upd)) 102 illustrated inFIG. 1, normal image updates and uPDDs are both achieved through thesame TFT (i.e., TFT_((upd))). Alternatively, in some embodiments, anadditional TFT may be added to each pixel and used solely for the uPDDdischarging scheme. While the overall discharging scheme remains thesame, the pixel TFT (e.g., TFT_((upd)) 102 of FIG. 1) that is used fornormal display operation will be used only for active display updates,just like in standard active-matrix driving of EPDs that do notincorporate the discharging. This configuration ensures that theperformance of the pixel TFT used for normal display operation is stableand unaffected by the discharging. While the additional TFT used fordischarging may experience threshold voltage shift due to positive biasstress but this will not cause optical shifts in the EPD, and this willnot affect the discharging operation as long as the TFT is turned onduring discharging (i.e., as long as the potential threshold voltageshift is account for by the discharging scheme). Such configuration canallow for stable display operation without optical response shifts whileat the same time allowing for DC-imbalanced waveforms as enabled bypost-drive discharging.

One exemplary embodiment in accordance with the concept described aboveis illustrated in FIG. 3. In addition to the standard pixel TFT (e.g.,TFT_((upd)) 302), a display pixel 300 may include an active componentdedicated for draining the remnant voltage or excessive charges from theelectrophoretic film 314. This active component may be a transistor ofany kind (e.g., TFT, CMOS etc.) or any other component that may beactivated or turned on by an application of an electrical (e.g.,voltage) or optical energy, devices such as a diode or a photodetector/diode, or any electrically/optically activated switch ingeneral. For the purpose of illustrating the general concept, a TFT(e.g., an n-type ITT) is used herein, but it should be appreciated thatthis is not meant to serve as the limitation. As illustrated in FIG. 3,a designated transistor TFT_((upd)) 304 may be used for the purpose ofdischarging the charges of the remnant voltage within theelectrophoretic imaging film 314. In this configuration, the gate of theTFT_((upd)) 302 is connected to the select lines (e.g., Vg(upd) 308)from the gate driver outputs, while the gate of the TFT_((dis)) 304 isconnected to a discharge select line such as the Vg(dis) 306, where thisselect line may be used to turn on and off the TFT_((dis)) 304 at itsgate (e.g., by supply a voltage to the transistor's gate through theselect line to affect the gate-source or gate-drain potential). In oneembodiment, all the pixel discharge select lines for multiple pixels maybe connected together to a single display output such as to turn on allthe pixel discharge TFT (e.g., TFT_((dis)) 304) transistors of all thedisplay pixels of a display at the same time for simultaneousdischarging of the whole display. In some embodiment, the source linesof the TFT_((upd)) 302 and the TFT_((dis)) 304 may be both connected tothe data lines Vs 310. During operations, the TFT_((dis)) 304 may beturned off for all the pixels while the TFT_((upd)) 302 is used foractive updating of the display. During discharging, the TFT_((dis)) 304can be turned on while the TFT_((upd)) 302 may be turned off. In someembodiments, either or both the TFT_((upd)) 302 and TFT_((dis)) 304 maybe an n-type transistor. In that case, the source of the TFT_((upd)) 302may be electrically coupled to the source line Vs 310, and the drain ofthe TFT_((upd)) 302 may be coupled to the pixel electrode 312 of thedisplay pixel 300. In addition, if the TFT_((dis)) 304 transistor is ann-type transistor, its source may be coupled to the source line Vs 310,while its drain may be coupled to the pixel electrode 312. In practice,when the TFT_((dis)) 304 is turned on and conducting, charges from theelectrophoretic film 314 may be drained or discharged through theTFT_((dis)) 304 and/or the source line Vs 310.

FIG. 4 illustrates another embodiment of a display pixel 400 inaccordance with the subject matter presented herein. In this embodiment,a discharge TFT_((dis)) 402 may be electrically coupled to an EPD's topplane 404 (e.g., connected to the ETD's common electrode) and the Vcom406 voltage line as shown in FIG. 4 (e.g., the discharge TFT's 402 drainis directly coupled to the EPD's top plane 404, while its source iscoupled to pixel's pixel electrode 408). In this configuration, thedischarging of the display module does not occur through the sourcedrivers (e.g., Vs 410) but instead is done directly through the topplane connection. In addition, with this setup, it is possible todischarge the display during an update by putting the dischargeTFT_((dis)) 402 in a weakly conductive state as to act as a resistive orconductive path for discharging, because the Vs 410 is not connected tothe discharge TFT_((dis)) 402 in this case and therefore does notinfluence its operation. In this configuration, the TFT_((dis)) 402 maybe activated through the select line Vg_((dis)) 412, while thetransistor TFT_((upd)) 414 may be activated by through the select lineVg_((upd)) 416, where the two select lines Vg_((dis)) 412 and Vg_((upd))416) may be optionally not electrically coupled.

FIG. 5 illustrates an exemplary voltage sequence that may be applicableto either of the two proposed pixel designs presented in FIGS. 3 and 4.This voltage sequence ignores potential RC time constraints that mayappear when switching from one voltage to another or that may beintroduced during power down for example. Vg_((upd)) is connected to theselect line, as in standard active-matrix driving, switching between ahigh and a low voltage to turn on and off the TFT. During the activeupdate, Vcom may be held constant at a voltage that is typically equalto the kickback voltage of TFT_((upd)). Vs is connected to the data linethat provides the data signal to refresh the pixel with the desiredwaveform. Vg_((dis)) is connected to a low voltage in order to keepTFT_((dis)) turned off. During discharging after the active update,Vg_((upd)) is turned off, and Vcom and Vs are held at 0V Vg_((dis)) isturned on in order to short-circuit the electrophoretic imaging filmthrough TFT_((dis)). The voltage sequence shown in FIG. 5 is anexemplary illustration of the discharging scheme using the new TFT pixeldesign. This new TFT pixel design is flexible enough to accommodate morecomplicated implementations of the discharging scheme. The main idea isthat the discharging happens by turning on a dedicated TFT while leavingthe pixel TFT used for normal display operation out of the dischargingoperation. Secondary effects may include the possibility that thekickback voltage experienced by the TFT_((dis)) when turning off at theend of discharging may affect the discharging efficacy or opticalperformance of the display. Such effects may be mitigated byimplementing properly designed power-down circuitry for Vg_((dis)) withcertain RC decay in order to prevent or minimize such effects.

In the description above, the TFT_((upd)) and TFT_((dis)) are bothN-type TFTs. These transistors could also be both P-type TFT or N-typeand P-type each. One of the example based on the circuit in FIG. 3 isshown in FIG. 6, where both the TFT_((upd)) 604 and TFT_((dis)) 602 areP-type TFTs. The same could be done for the circuit in FIG. 4 (not shownhere).

Alternatively, instead of an active component such as a TFT, passivecomponents can also be adopted to discharge the EPD. FIG. 7 showsanother possible implementation of the subject matter presented hereinwhere a resistor R_(dis) 702 is placed in parallel with the storagecapacitor Cs 704 of the pixel. As shown, resistor R_(dis) 702 is alsocoupled to both the pixel electrode 706 and the common electrode 708.The purpose of this resistor is to provide a pathway to discharge theremnant voltage from the electrophoretic imaging film at the end of adriving period. The benefit of this pixel design is that it does notrequire adding an extra line Vg_(dis) to control the second TFT.However, since R_(dis) 702 now has a fixed resistance value, theresistance value of R_(dis) 702 needs to be designed appropriately. Forexample, the RC constant R_(dis) 702 associated with the addition ofR_(dis) 702 to the pixel circuit needs to be larger than the drivingframe time in order to achieve the required pixel voltage holdingcharacteristics during the frame time. This RC constant also needs to below enough to provide sufficient discharging at the end of the drivingperiod. In some other embodiments, the R_(dis) 702 may also be replacedwith a field switchable shunt resistor using amorphous silicon or anyother technologies that provide an appropriate resistance in parallelwith the electrophoretic imaging film for discharging without preventingnormal driving operation.

In addition to providing a dedicated TFT used only for discharging, andanother TFT used only for display update in order to avoid opticalshifts in the display performance due to positive bias stress, thesubject matter presented herein also enables some additional usage modesthat could be beneficial as described below

FIG. 8 shows an exemplary voltage sequence applicable only to thecircuitry presented in FIG. 3 where the TFT_(upd) 302 and TFT_(dis) 304have dedicated gate lines. in this voltage sequence, the TFT_(upd) 302and the TFT_(dis) 304 are both turned on during the active update stage,while the TIFT_(dis) 304 may or may not be turned on at the end of theupdate for discharging. in this usage mode, the TFT_(dis) 304 couldprovide extra current for faster pixel charging that could enable forexample higher frame rate driving. Furthermore, the TFT_(dis) 304 inproposed pixel designs can also be used as a global update transistor.By turning on TFT_(dis) 304 and turning off TFT_(upd) 302, we couldprevent long term positive bias on TFT_(upd) 302 when the global updateis performed.

As such, the subject matter presented herein introduces a way to enableDC-imbalance waveforms by making use of post-drive discharging and itsvarious implementations without creating a positive bias stress on thepixel TFT used for standard active-matrix driving display operations.This results in more stable display optical response while at the sametime enabling post-drive discharging.

It will be apparent to those skilled in the art that numerous changesand modifications can be made to the specific embodiments of theinvention described above without departing from the scope of theinvention. Accordingly, the whole of the foregoing description is to beinterpreted in an illustrative and not in a limitative sense.

1. A thin film transistor (TFT) backplane comprising a plurality ofelectrodes, each of the plurality of electrodes being coupled tocircuitry comprising: a first thin film transistor OTT coupled to theelectrode for transmitting waveforms to the electrode; and a second TFTcoupled to the electrode for discharging remnant charges from theelectrode; wherein the second TFT is activated subsequent to the firstTFT being deactivated.
 2. The TFT backplane of claim I furthercomprising a first select line coupled to the first TFT for activatingthe first TFT.
 3. The TFT backplane of claim 2 further comprising asecond select line coupled to the second TFT for activating the secondTFT, wherein the second select line and the first select line are notelectrically connected.
 4. The TFT backplane of claim 1 wherein thefirst TFT and the second. TFT are n-type TFTs.
 5. The TFT backplane ofclaim I. further comprising a common electrode and an electrowettingmaterial positioned between the common electrode and one of theplurality of electrodes.
 6. The TFT backplane of claim 5 wherein thedrain of the first TFT and the drain of the second TFT are coupled tothe common electrode.
 7. A method for discharging remnant voltage from aTFT backplane, the TFT backplane comprising a plurality of electrodes,each of the plurality of electrodes being coupled to circuitrycomprising: a first thin film transistor (TFT) coupled to the electrodefor transmitting waveforms to the electrode; and a second TFT coupled tothe electrode for discharging remnant charges from the electrode, themethod comprising: activating the first TFT; deactivating the first TFT;and activating the second TFT, subsequent to the deactivation of thefirst TFT, to discharge remnant charges from the electrode.
 8. Themethod of claim 7, wherein the activating the second TFT to dischargeremnant charges further comprises discharging the charges from anelectrowetting material.